Various systems for optical data transmission from a remote terminal to optical ground terminals are known and deployed. The primary function of these systems is to reliably transmit data from a data source to an optical ground terminal.
However, line-of sight communication contact with the optical ground terminal is limited during each flyover. Therefore the achieved speed of downlink channel is of upmost importance, since a great amount of data has to be transmitted to the optical ground terminal in a short amount of time for which a flyover lasts.
When scaling the downlink data rate from remote terminals to Earth, using optical frequencies, the required technical complexity, mass, power and volume for one on-board laser communication terminal cannot be smoothly upgraded, but it rather undergoes a technology-step once transmitter telescope diameters get so large that the required line-of-sight steering of the optical downlink beam cannot be achieved by a single high-precision actuator. At that technology step, nested control loops are required, usually comprising multi-axis course—and fine steering actuators together with complex optical beam routing and different kinds of optical sensors, altogether building up toward a complex on-board laser communication terminal that weighs several tens of kilograms and that requires dedicated satellite structural support for accommodation of large diameter telescopes.
It is known that mass increases with the telescope diameter ratio raised to approximately 3rd power, thereby inherently leading to non-linear increase in involved launch cost for the spacecraft operator who wants to employ laser communication terminals.
Examples for larger laser communication terminals are available from space demonstration missions like SILEX, TerraSAR-X. Even though quasi-stationary, Alphasat communication satellites—even though not that limited by reduced line-of-sight times—also face the requirement of increased downlink capacity. In direct-to-Earth link scenarios, such large optical terminals that link to optical ground stations are costly because they include highly sophisticated technologies, in order to exploit to maximum extent all capabilities of costly larger space laser communication terminals, clearly for economical reasons.
Furthermore, optical communication devices -as any other complex systems—are prone to failure thereby making them less reliable. In addition, optical links have the further disadvantage that even in relatively good weather, the optical line-of-sight might be disturbed thereby interrupting communication and thus degrading the availability of the system. However, since the line-of sight communication contact is broken after a short amount of time of a flyover and can be established again only at the next flyover after a further revolution (or not even), the reliability of the transmission is essential.
The problem arising with above described technical features is to achieve lower technical entry levels for laser communication system usage that allow users to increase the laser communications capabilities according to their needs. Today this leads to design constraints tailored to “mission-only”, involving each time high amounts of repeated development effort.